Air Conditioning System and Method For Avoiding Condensation of Humidity on Vehicle Windows

ABSTRACT

An air-conditioning system in a vehicle comprises an air-temperature sensor for measuring the interior temperature, an air-conditioning unit and a regulating and control unit for regulating the air-conditioning unit. Moreover, a moisture sensor for measuring the air humidity and a window-temperature sensing device for determining the window temperature of a vehicle window are provided. The air-conditioning unit is actuated in the case where the difference between the window temperature and the dew-point temperature undershoots a limiting value.

The invention relates to an air conditioning system in a vehicle as recited in the preamble to claim 1 and to a method for avoiding condensation of humidity on vehicle windows as recited in claim 10.

DE 199 32 691 A1 has disclosed a method and apparatus for air conditioning an interior of a motor vehicle. In order to prevent the interior of the vehicle from heating to an impermissible degree while the vehicle is parked, during operation of the air conditioning system when the vehicle is driving, the accumulated condensation water is collected and the supply air that is supplied to the interior or the exhaust air that is discharged from the interior is humidified with the collected condensation water in order to reduce the interior temperature using the evaporative cooling effect. Decisive influence variables for the operation of the apparatus include the outside temperature, the vehicle interior temperature, and the desired interior temperature.

As a rule, when the air conditioning system is activated, the humidity inside the vehicle is reduced at the same time, which is advantageously accompanied by a simultaneous reduction in the tendency for condensation to form on the inside, in particular, of the windshield. This reduction in the condensation tendency,

however, is only a side effect of air conditioning the vehicle interior; this process does not permit selective prevention of condensation from forming on the vehicle window.

OBJECT AND ADVANTAGES OF THE INVENTION

The object of the present invention is to reliably prevent the condensation of moisture on vehicle windows through simple means.

This object is attained with an air conditioning system according to the invention that has the defining characteristics of claim 1 and a method according to the invention that has the defining characteristics of claim 10. Suitable modifications are disclosed in the dependent claims.

In addition to the air temperature sensor for measuring the interior temperature, the air conditioning system that is used to prevent the condensation of moisture also has a humidity sensor for measuring the humidity of the interior. The system is also provided with a window temperature determining device that serves to determine the window temperature of one of the vehicle windows. Based on the measured and/or determined values of the interior temperature, humidity of the interior, and window temperature, it is possible, based on stored tables and calculation guidelines, for a regulation and control unit to determine the dew point temperature that is associated with the determined interior temperature and humidity; in an additional step, the difference between the window temperature and dew point temperature is compared to a threshold value and the air conditioner in the vehicle is activated if this difference falls below a threshold value. This falling below the threshold value can be interpreted as an increase in the probability of condensation forming on the vehicle window; the setting of the threshold value influences the tolerance range. A relatively high value of the threshold value means that even relatively large temperature differences between the window temperature and the dew point temperature are sufficient for the air conditioner in the vehicle to be activated. Since the dew point temperature is below the interior temperature and, as a rule, the window temperature is between the dew point temperature and the interior temperature, the difference between the window temperature and the dew point temperature represents the safety margin before moisture will begin to condense on the vehicle window. The condensation first occurs when the window temperature falls to the dew point temperature. This can be reliably prevented, however, by taking prompt countermeasures.

These countermeasures executed to prevent the condensation of moisture largely involve activating an air conditioner provided in the vehicle. This air conditioner has a blower, a vehicle heater, and/or an air conditioning unit in the vehicle. To prevent the condensation of moisture, on the one hand the humidity inside the vehicle can be reduced and on the other hand, the temperature can be increased; it is useful to execute both steps simultaneously.

The determination of the dew point temperature associated with the measured interior temperature preferably occurs in two steps. First, the saturation pressure—which is associated with the measured interior temperature and lies on the saturated vapor curve—is determined, for which it is possible to consult known tables or a known functional relationship, which is in particular stored in the regulation and control unit. Then, based on the determined saturation pressure and the measured humidity inside the vehicle and in accordance with a likewise known functional relationship that is also stored in the regulation and control unit, the dew point temperature is calculated, which will be used subsequently for comparison with the window temperature.

It is advantageous if the vehicle window on which the condensation of moisture is to be prevented is the windshield of the vehicle. However, the method can also be applied to all other vehicle windows, i.e. to the side windows and the rear window. In order for the countermeasures for preventing moisture condensation to be as efficient as possible, the air conditioner in the vehicle is suitably activated in such a way that an advantageously preheated flow of air is aimed directly at the relevant window. It is useful for the flow of air to be directed against the inside, but it is also possible for a heating of the window to occur from the outside or internally, for example if heating wires are situated in or on the window.

The window temperature is advantageously measured on the inside of the window. In addition, however, it is also possible for the outside window temperature to form the basis underlying the air conditioning system and the method for avoiding moisture condensation. For example, an infrared temperature sensor can be used as the window temperature determining device, which on the one hand, offers the advantage that a sensor of this kind functions in a contactless fashion and on the other hand, is accompanied by a rapid reaction time so that countermeasures for preventing condensation of moisture can be initiated more quickly. As an alternative to the contactlessly operating infrared sensor, however, it is also possible to use other temperature sensors such as thermoelements or heat sensors with a quartz crystal oscillator.

According to another advantageous embodiment, the air conditioning system according to the invention can be a component of a rain sensor module suitably situated inside the vehicle, for example on the rearview mirror, and has a contactlessly operating sensor for determining when the outside of the window is wet with water. The sensors and the regulation and control unit of the air conditioning system can be integrated into the rain sensor module.

Other advantages and suitable embodiments can be inferred from the remaining claims, the description of the drawings, and the drawings themselves.

DRAWINGS

FIG. 1 is a schematic depiction of an air conditioning system in a motor vehicle, which is used for avoiding condensation of moisture on the windshield,

FIG. 2 is a pressure/temperature graph with several curves for various humidity levels; as an example, a measured interior temperature has been plotted on the graph, with an associated saturation vapor pressure and dew point temperature,

FIG. 3 shows a flow chart for carrying out the method for avoiding condensation of moisture on the vehicle window.

EXEMPLARY EMBODIMENT

The air conditioning system 1 schematically depicted in FIG. 1 is used in a motor vehicle and serves to prevent condensation of moisture, in particular on a windshield 2, and possibly also on other windows in the vehicle. The air conditioning system 1 has a plurality of sensors: an air temperature sensor 3 that measures the interior temperature T_(air), a humidity sensor 4 that measures the humidity φ inside the vehicle, and an additional sensor 5 that measures the inside window temperature T_(window) of the windshield 2. The sensor 5 for measuring the window temperature has the capacity to measure in a contactless fashion and is in particular embodied in the form of an infrared temperature sensor, which establishes a temperature measuring zone on the inside of the windshield 2. In addition to the advantage of a contactless measurement, a sensor of this kind also has a very short reaction time.

In addition, the air conditioning system 1 is associated with a regulation and control unit 7, which is supplied with input signals in the form of the values measured by the sensors 3, 4, and 5 and which—in accordance with stored functional or tabular relationships—generates output signals or actuation signals for activating an air conditioner 8 that is also a component of the air conditioning system 1. This air conditioner 8 serves to influence the physical state variables of the windshield 2 so as to prevent the condensation of moisture on the inside of the windshield 2. The air conditioner 8 suitably includes a blower in the vehicle, a vehicle interior heater, and/or an air conditioning system in the vehicle.

The thermodynamic relationships that form the basis underlying both the air conditioning system and the method for avoiding condensation of moisture on the vehicle window will be explained in conjunction with FIG. 2. Plotted by way of example in the pressure/temperature graph according to FIG. 2 is a point 1, which indicates the current interior pressure at the measured interior temperature T_(air) and humidity φ, which in the exemplary embodiment, lies at point 1, where φ=0.6. Starting from this point 1, if one follows an isobar to point 2 at the same pressure on the saturated vapor curve with a humidity of φ=1; the point 2 is thus associated with the dew point temperature T_(dew). The saturated vapor curve with φ=1 indicates the state in which the condensation forms. For example, if the window temperature T_(window) lies below the value of the dew point temperature T_(dew), then as depicted at point 3, condensation forms on the windshield. On the other hand, the condensation of moisture can be prevented if the window temperature T_(window) lies between the interior temperature T_(air) and the dew point temperature T_(dew), as depicted at point 4.

As depicted in FIG. 3, the method for avoiding moisture condensation on the vehicle windows proceeds as follows:

In a first method step V1, a measurement is first executed to determine the interior temperature T_(air) in the vehicle, the humidity φ, and the window temperature T_(window). These values are supplied to the regulation and control unit for further processing. In a subsequent method step V2, based on the measured values, established relations are used to determine the saturation pressure p_(sat), which is associated with the interior temperature T_(air) and lies on the saturated vapor curve φ=1 (FIG. 2), in accordance with the relation:

p _(sat)(T _(air))=f(T _(air))

This known relationship can be stored in the form of a value table or as a function equation in the regulation and control unit.

In the next method step V3, the dew point temperature T_(dew) is determined as a function of the measured humidity φ and the previously determined saturation pressure P_(sat)(T_(air)) in accordance with the following relation:

${T_{dew} = \frac{C_{1}*\left( {{\ln \left( {\phi*{p_{sat}\left( T_{air} \right)}} \right)} - C_{2}} \right)}{C_{3} - {\ln \left( {\phi*{p_{sat}\left( T_{air} \right)}} \right)}}},$

where C₁ is a constant with the value 243.12, C₂ is a constant with the value 1.81, and C₃ is a constant with the value 19.43.

In the next method step V4, the query

T _(window) −T _(dew) <T _(threshold)

is executed, where the threshold temperature T_(threshold) is stored as a previously established value in the regulation and control unit. The threshold temperature T_(threshold) can either be a fixed, unchanging value or can also be determined based on state and system variables and be continuously updated. The threshold temperature T_(threshold) represents a safety margin to the extent that countermeasures for preventing the condensation of moisture on the vehicle window are initiated even when the window temperature T_(window) still lies above the dew point temperature T_(dew) by the value T_(threshold).

If the result of the query in method step V4 is that the difference between the window temperature and dew point temperature does not lie below the threshold temperature, then the process follows the “no” branch back to the first method step and the entire sequence starts over, without requiring special countermeasures to be taken to prevent the condensation of moisture. In this case, the difference between the window temperature T_(window) and the dew point temperature T_(dew) is still great enough that there is no danger of condensation occurring.

However, if the result of the query in method step V4 is that the temperature difference has fallen below the threshold value T_(threshold), then the process continues along the “yes” branch to the next method step V5, according to which air conditioning actions are executed, for example the blower is activated, the interior temperature is increased, or heating wires in the relevant window are heated. After this, the process returns to the first method step V1 and the whole sequence is repeated in cyclical fashion.

As an alternative to the query, shown in method step V4 in FIG. 3, as to whether the difference between the window temperature T_(window) and the dew point temperature T_(dew) is less than the predetermined threshold temperature T_(threshold), it is also possible to execute a query as to whether a condensation tendency κ, which is to be determined as a function of the window temperature T_(window) and the dew point temperature T_(dew) based on the relation

${\kappa = \frac{1}{T_{window} - T_{dew}}},$

exceeds a threshold value κ_(threshold):

κ>κ_(threshold).

If this is so, then after method step V4, the process continues on to the next method step V5 and the air conditioning is activated; otherwise, the process is interrupted in accordance with the “no” branch and goes back to the beginning of the sequence. Like the temperature threshold value T_(threshold), the threshold value κ_(threshold) to be compared to the condensation tendency κ can either be stored as a fixed, unchanging value or can be determined continuously based on state and operating variables. 

1. An air conditioning system in a motor vehicle, having an air temperature sensor (3) for measuring the interior temperature (T_(air)) in the vehicle, an air conditioner (8), and a regulation and control unit (7) for regulating the air conditioner (8) as a function of the measured interior temperature (T_(air)), wherein a moisture sensor (4) for measuring the humidity (up) inside the vehicle and a window temperature determining device (5) for determining the window temperature (T_(window)) of the vehicle window (2) are provided, based on the measured interior temperature (T_(air)) and humidity (φ), it is possible for the associated dew point temperature (T_(dew)) to be determined in the regulation and control unit (7), and the air conditioner (8) is activated if the difference between the window temperature (T_(window)) and dew point temperature (T_(dew)) falls below a threshold value (T_(threshold)).
 2. The air conditioning system as recited in claim 1, wherein in the regulation and control unit (7), the associated saturation pressure (p_(sat)(T_(air))) lying on the saturated vapor curve is determined based on the interior temperature (T_(air)) and the dew point temperature (T_(dew)) is determined as a function of the humidity (φ) and the saturation pressure (p_(sat)(T_(air))).
 3. The air conditioning system as recited in claim 2, wherein the dew point temperature (T_(dew)) is determined as a function of the humidity (φ) and the saturation pressure (P_(sat)(T_(air))), based on the following relation: ${T_{dew} = \frac{C_{1}*\left( {{\ln \left( {\phi*{p_{sat}\left( T_{air} \right)}} \right)} - C_{2}} \right)}{C_{3} - {\ln \left( {\phi*{p_{sat}\left( T_{air} \right)}} \right)}}},$ where C₁ is a constant with the value 243.12 C₂ is a constant with the value 1.81 C₃ is a constant with the value 19.43.
 4. The air conditioning system as recited in claim 1, wherein the window temperature determining device is embodied in the form of an infrared temperature sensor (5).
 5. The air conditioning system as recited in claim 1, wherein the air conditioner (8) is activated if the condensation tendency (κ), which is determined as a function of the window temperature (T_(window)) and the dew point temperature (T_(dew)) based on the relation ${\kappa = \frac{1}{T_{window} - T_{dew}}},$ exceeds a threshold value (κ_(threshold)).
 6. The air conditioning system as recited in claim 1, wherein the air conditioner (8) includes an air conditioning unit.
 7. The air conditioning system as recited in claim 1, wherein the air conditioner (8) includes a blower.
 8. The air conditioning system as recited in claim 1, wherein the air conditioner (8) includes a heater.
 9. A rain sensor module equipped with an air conditioning system as recited in claim
 1. 10. A method for avoiding condensation of moisture on vehicle windows, in particular a method for operating the air conditioning system (1) as recited in claim 1, in which the interior temperature (T_(air)), the humidity (φ) in the vehicle, and the window temperature (T_(window)) of a vehicle window (2) are measured or determined and the interior is air conditioned if the difference between the window temperature (T_(window)) and the dew point temperature (T_(dew)) falls below a threshold value (T_(threshold)), with the dew point temperature (T_(dew)) being determined as a function of the humidity (φ) and the interior temperature (T_(air)). 